Two-phase fluid pumping covers a large spectrum of pump operation and applications. In certain situations the entrained gas within a liquid medium will cause unwanted problems in the pumping process. For the off-shore oil industry there is now considerable interest in pumping liquids with a high gas content, similar as there has been for some time in the pumping systems supplying aircraft gas turbines. This interest is also found within the geothermal industry. In many oil fields, the wells contain mixtures of gas and oil in varying proportions. The handling of such fluids can create problems. The problems are strictly multi-phase, which essentially means that as the gas or steam content increases the pressure head degrades multi-phase pumps which have relatively good performance characteristics are normally of screw type. While centrifugal pumps are known to be used in the pumping of multi-phase fluids they have had limited success. It is however desirable for centrifugal pumps to be utilised in the pumping of fluids as centrifugal pumps provide the benefit of both a reduced cost when compared to many other types of pumps, simplicity of operation and construction and hence reduced maintenance, and also are normally of a smaller size.
Two-phase pumping applications utilising a centrifugal pump are known in for example the pumping of sewage. Flow separation of the different fluids prior to pumping of the pump is a common way of dealing with delivery of multi-phase flow. This is for example illustrated in the specification of U.S. Pat. No. 5,580,214. The present conventional designs of centrifugal pumps are not adequate due to their inability to pump high gas volume fraction media. Work by A Furukawa: “Fundamental studies on tandem blades impeller of gas liquid two phase centrifugal pump” Memoirs of the Faculty of Engineering Kyushu University, 48, 4, 231–40 or “On an improvement in air/water two phase flow performance of a centrifugal pump in the partial flow rate of water.” 69th JSME Fall Annual Meeting, vol. B, paper number 1118, pp. 165–7[1] suggests using tandem or slotted blades to reduce the degradation in a limited range, of liquid to gas content.
Two-phase flow includes both a compressible and substantially incompressible fluid and the coexistence of liquid and vapour phases. The composition of the fluid flow and variety of flow configurations where each phase has a different velocity in such cases makes the flow difficult to define. Particularly as the flow composition and configurations vary over time, and may reach gas volume percentages as high as 90 to 100 percent.
It is known that where free gas is present in a liquid being pumped the head, power and efficiency of rotodynamic pumps are known to decrease. (See FIG. 1). Although multi-phase products have been pumped for many years with double screw type pumps, centrifugal impeller pumps have shown serious departures from published performance curves. The head when surging begins, oscillates from high to low values once the percentage by volume of gas exceeds some point between 7% and 11% by volume of intake.
The mechanism that seems to control surge and chocking in a centrifugal pump which is pumping a multi-phase fluid is the separation of the gas phase from the liquid phase and a tendency to coalescence in a large gas pocket at the blade entry throat and the sonic chocking to the reduction of the speed of sound. The various pressure fluids which operate inside the impeller passages play a critical role in the two mechanisms mentioned above.
The decrease in efficiency of pumping multi-phase flow suggests that some additional loss mechanisms arise when gassy liquids are pumped. The decrease in head is greater than that which can be associated with the decrease in average density of the liquid-gas mixture. The pump performance decreases continuously as the gas volume increases until at a certain critical gas content the pump loses prime. The above trend is common to radial, mixed, and axial-flow type pumps either in single or in multistage configurations.
Basically the operating range appears to be limited by two phenomena:
1) gas locking or “choking”, and
2) instability in the head-capacity curve which causes surge.
Also the property of two-phase flow media is the large influence of gas content on the speed of sound. Normal values of peak relative velocities around the blade leading edge are in this range. Therefore, pumps operate at transonic or supersonic local flow. It is not surprising that a blade design for incompressible single-phase flow is not very effective in multi-phase flow and produces choking for relatively high gas volume fraction. Theory shows that the dramatic variation of the speed of sound at low percentages is very much related to the large difference in density between the two phases. The sonic velocity in two-phase mixtures is also pressure dependent.
Accordingly it is an object of the present invention to reduce the abovementioned disadvantages and problems or to at least provide the public with a useful choice.